1. Field of the Invention
This invention relates to an encoder, and particularly to an encoder for detecting the relative displacement of a diffraction grating and a light beam incident on the diffraction grating by photoelectrically converting an interference light formed by causing several diffracted lights emerging from the diffraction grating to interfere with one another.
2. Related Background Art
Heretofore, in an NC machine tool or the like, an encoder has been used as a sensor for detecting the position or the angular displacement of an object to be examined. In recent years, a high resolving power and high accuracy have been required of encoders of this type.
An encoder of high resolving power and high accuracy in which a diffraction grating is used as an optical type scale for displacement detection and the recording density of the diffraction grating is several microns/pitch and several diffracted lights emerging from the diffraction grating are caused to interfere with one another to thereby obtain a periodic signal conforming to the displacement of the scale is already known. However, for higher accuracy and higher resolving power, the recording density of the diffraction grating is increased to the wavelength order, the angle of diffraction (the angle of emergence from the diffraction grating) of the diffracted lights will become greater, and this gives rise to a problem that the arrangement of optical parts become cumbersome.
For example, a prior-art encoder shown in FIG. 15 of the accompanying drawings generates a signal conforming to the displacement of a scale by an operation as will be described hereinafter.
A light beam from a laser diode 1 is collimated by a collimator lens 2 and is caused to be vertically incident on a point Pl on a light reflecting type diffraction grating 5, and +1st-order reflected diffracted light (R1+) emerging from the point P1 is returned to a beam splitter 4 via a mirror 61 and at the same time, -1st-order reflected diffracted light (R1-) emerging from the point P1 is returned to the beam splitter 4 via a mirror 62, and .+-.1st-order reflected diffracted lights are superposed one upon the other through the beam splitter 4 and are caused to interfere with each other. By the principle that while the diffraction grating 5 is moved by an amount corresponding to 1 grating pitch, the phase of the wave surface of +1st-order diffracted light is "advanced by 2.pi." and the phase of the wave surface of -1st-order diffracted light is "delayed by 2.pi.", a variation in light and shade of two periods is observed by a light receiving element S in conformity with the movement by 1 grating pitch by virtue of an interference light formed by causing the two lights to interfere with each other. That is, a periodic signal double the number of the gratings of the diffraction grating 5 can be taken out.
However, as previously described, the greater the recording density (pitch) of the diffraction grating 5 becomes, the greater the angle of diffraction of the diffracted light becomes and therefore, the angle of emergence of the diffracted light emerging from the diffraction grating 5 becomes approximately 90.degree.. Accordingly, mirrors 61 and 62 must be installed proximate to the diffraction grating 5 so as not to be in contact with the diffraction grating 5, and such installation is very cumbersome. Further, if the grating pitch of the diffraction grating 5 is made less than the wavelength of the light beam from the laser diode 1, the diffracted light cannot be taken out and it becomes impossible even to detect any variation in the diffraction grating 5.
A linear encoder and a rotary encoder are known as the above-described encoder, and a conventional example of the rotary encoder will hereinafter be described. The rotary encoder is such that a diffraction grating is provided on the circumference of a disk (scale) connected to a rotational body, a laser light is applied to the diffraction grating, diffracted lights produced by the diffraction grating are caused to interfere with each other, and a variation in the light and shade of the interference light is detected to thereby detect the angle of rotation and the speed of rotation of the disk.
FIGS. 16A and 16B of the accompanying drawings show the construction of a prior-art rotary encoder described in Japanese Laid-Open Patent Application No. 63-91515. In these figures, the reference numeral 1 designates a laser, the reference numeral 2 denotes a scale having a light transmitting type diffraction grating, the reference numeral 3 designates a reflecting prism, the reference numeral 4 denotes a polarizing prism, the reference numerals 51 and 52 designate light receiving elements, and the reference numeral 6 denotes the rotary shaft of the scale 2. In FIG. 16A, a light beam emitted from the laser 1 is substantially vertically incident on a position M1 on the diffraction grating 2. .+-.1st-order diffracted lights created at the position M1 are reflected at a right angle by a first right-angled reflecting surface 3a of the reflecting prism 3, and are totally reflected twice by the sides 3c and 3d of the reflecting prism 3, and thereafter are again reflected at a right angle by a second right-angled reflecting surface 3b of the reflecting prism 3 and are caused to be incident on a position M2 on the diffraction grating 2. FIG. 16B illustrates the optical path in the reflecting prism 3, and is a plan view as it is seen from the underside of FIG. 16A. As shown in FIG. 16B, .+-.1st-order diffracted lights created at the position M1 emerge at angles of diffraction .alpha.+ and .alpha.-, respectively, and are totally reflected by the sides 3c and 3d of the reflecting prism 3. They intersect each other near the center of the reflecting prism 3, are further totally reflected by the sides 3c and 3d, and are incident on a position M2 on the diffraction grating 2 at the same angles as the aforementioned angles of diffraction .alpha.+ and .alpha.-. Thereupon, the .+-.1st-order rediffracted lights at the position M2 are superposed one upon the other in parallel to and in the opposite direction to the incident light from the laser 1 at the position M1, and emerge from the diffraction grating 2. These .+-.1st-order rediffracted lights which have interfered with each other are received by the light receiving elements 51 and 52 through the polarizing prism 4. The .+-.1st-order diffracted lights have their phases varied by .+-.2.pi. when the diffraction grating 2 is rotated by an amount of 1 grating pitch. Likewise, the .+-.1st-order rediffracted lights have their phases varied by .+-.4.pi. by the rotation of 1 grating pitch. Accordingly, when as shown in FIGS. 16A and 16B, the .+-.1st-order rediffracted lights are caused to interfere with each other, a sine wave signal corresponding to four periods is obtained from the light receiving elements 51 and 52 for the rotation of the scale 2 by an amount corresponding to 1 grating pitch. If the total number of the gratings is N, a sine wave signal corresponding to 4N periods is obtained for one full rotation. In FIGS. 16A and 16B, M1 and M2 are in a positional relation point-symmetrical with each other with respect to the center of rotation of the rotary shaft 6, whereby no measurement error will occur even if there is some eccentricity during the mounting of the scale 2 onto the rotary shaft 6. Further, from the light receiving elements 51 and 52, 90.degree. phase difference signals are obtained by a combination of the rectilinearly polarized light of the laser 1, the elliptically polarized light by the total reflection in the reflecting prism 3 and the polarizing prism 4 so that the direction of rotation of the diffraction grating 2 can also be discriminated.
In the example of the prior art constructed as described above, the problem as previously described arises if the pitch of the grating is made fine. In addition to this, the following problems also arise:
(1) The .+-.1st-order diffracted lights which are caused to interfere with each other pass along discrete optical paths in the reflecting prism 3. Therefore, the optical path is made to deviate or the length of the optical path is varied by an environmental change such as a change in the ambient temperature and a measurement error is liable to occur. Particularly, the greater the diameter of the scale 2 becomes, the longer the optical path in the reflecting prism 3 becomes, i.e., the non-common optical path, and an error is more liable to occur. Also, if the oscillation wavelength of the laser varies for such an environmental change, the optical paths of .+-.1st-order diffracted lights will vary and the .+-.1st-order diffracted light will not be incident on the position M2.
(2) If the diffraction grating forming surface of the scale 2 is inclined relative to the rotary shaft 6, the diffracted lights created at M1 will not be again incident on M2 and therefore, a measurement error will occur under the influence of the aforementioned eccentricity.